1. Field of the Invention
The present invention relates generally to a wafer machining apparatus, and more particularly to a wafer machining apparatus for machining a semiconductor wafer in a chemical mechanical polishing (CMP).
2. Description of Related Art
A conventional semiconductor wafer machining apparatus is comprised mainly of a polishing part having a turntable, and a load stage. This wafer machining apparatus holds a wafer, which has not yet been machined, in a concave part of the load stage to hereby hold the centered (positioned) wafer. The load stage centers the wafer, so that the wafer can be transferred to the polishing part with the position being a reference and can be polished at a regular position. It is therefore important to center the wafer at the load stage in order to uniformly polish the surface of the wafer.
When the wafer is centered at the load stage, the load stage or a wafer holding part of the polishing part is moved so that they can become closer relatively to one another. The wafer holding part holds the wafer, held on the, load stage, by suction. After the wafer holding part is moved to the turntable, the wafer is polished in the state of being pressed by the wafer holding part against the turntable. That completes the polishing of one wafer.
In the conventional wafer machining apparatus, the wafer holding part holds the wafer, held (contained) in the concave part of the load stage, by suction. Thus, the wafer holding part cannot hold the wafer by suction because the wafer holding part comes into contact with the top of the concave part before holding the wafer by suction.
To solve this problem, the depth of the concave part is smaller than the thickness of the wafer, and the top of the wafer is projected from the top of the concave part. In this case, however, the wafer may be displaced from the concave part.
Therefore, it is preferable to form the concave part with a larger depth than the thickness of the wafer in order to hold the centered wafer. On the other hand, it is preferable to form the concave part with a smaller depth than the thickness of the wafer in order to hold the wafer with a chuck. Thus, there is a conflict between the two ideas.